If you're looking for the best low-light ability (within some form of 'reason'), currently you will likely want to look at a digital SLR will a full frame sensor. It will cost you a load.
I'm not sure how read up you are on the sensor varieties but (in case you are not) the big two are APS-C and Full Frame. All Pentax DSLRs currently have an APS-C sized sensor (which is smaller than Full frame). With Nikon or Canon, there are several different APS-C as well as Full Frame bodies available.
With Nikon or Canon there is also a vibrant second hand market out there for lenses and accessories.. it is MUCH easier to find these items locally in most places than it is to find them for Pentax second hand. For instance, go to your local craigslist and type in 'Nikon' then type in 'Canon'. Finally, type in 'Pentax' -- I suspect, if it is anything like my searches, you will find a healthy page for each Canon and Nikon and a very short list of ancient film bodies for Pentax on offer hehehe.
Pentax has a fantastic series of Prime (fixed focal length) lenses. And a decent set of Weather Resistant lenses to boot.
Canon I know at least does too but you pay through the nose for it.
If you are looking for an APS-C body, I don't think you can find a more modern one currently than the K-3 anywhere.
If you are looking for a Full Frame body, You currently won't find a Pentax one (and it is a huge hot button topic on these forums).
Pentax (a K-x) was my first DSLR several years ago after the suggestion of a friend (who then did a 180 and went with Canon!!!). I was originally going to get a Nikon body before he convinced me to look at Pentax.
And, in the past few years, I've considered jumping ship for Nikon several times and even Canon once or twice but I stubbornly keep upgrading Pentax bodies. Simply because they work fantastically. My K-x (and subsequent K-30) both accepted AA batteries and were pretty lightweight while feeling substantial. Low-light performance has been pretty good.
The Canon Rebel series of bodies (imo) are junk comparatively and the higher end ones from them cost a shed load.
Nikon (to me) has the most logical system going (to the point I bought my Dad a d3100 a couple years back when he was interested in an entry level DSLR). But the lower end bodies from them feel hollow and plasticy and I wasn't impressed with the overcooked color reproduction (at least on the d3100). I haven't tried a higher end body like a D5x00 series or better.
Currently, I'm reading a book on 'On-Camera Flash' and the book reads 'for the Canon system a tip when using your camera..' or 'for the Nikon system..' as if these are the only two systems available. I'm a novice in using flash though I've heard the Pentax system is a little out of date (someone please correct me if untrue) compared to the other two, so that might be something to look into as well perhaps?
But long ramble, short -- It really depends on what you want to do with your camera.
If you are looking to be a sports photographer? Maybe Pentax isn't the proper brand of camera for you; I don't think there are a ton of long, fast lenses available. But, If you are looking to take your camera out into the wilderness (rain sleet or snow)? MAybe a Pentax camera (such as the K-3 with a sealed lens) is right on target.
btw the Sigma lens you mentioned is coming in Pentax format
Sigma 18-35mm f/1.8 DC HSM Lens for Pentax 210-109 B&H Photo
Another idea (at least in my mind this makes sense hehe) is to go to a photo meet and talk to owners of various cameras to get an idea of what they think of their cameras... perhaps even convince one or two to let you shoot a little bit with them.
Biggest concern I have with Pentax is the accessory market is extremely slim. There are tons of L-Brackets, molded plates, extension tube sets, flashes and flash accessories, etc etc all available for Canon and Nikon and pretty affordable. But not so much for Pentax. It is somewhat aggravating. Not to mention, again, the fact that there are less people using Pentax means your chances of finding affordable gear second hand is much slimmer.
My friend just sold his old, well used Canon T1i locally in a heartbeat.. but me? I've been trying to sell a not-even-a-year-old, clean K-30 kit for two months with no results so far.
But that brings me to another point.. Pentax camera bodies seem to drop in value fast compared to Nikon or Canon. But you can play this to your advantage.